<rdar://problem/13221060>

Make register read and write accept $<regname> as valid.
This allows:
(lldb) reg read rbx
     rbx = 0x0000000000000000
(lldb) reg read $rbx
     rbx = 0x0000000000000000
(lldb) reg write $rbx 1
(lldb) reg read $rbx
     rbx = 0x0000000000000001

to function correctly

It is not done at the RegisterContext level because we should keep the internal API clean of this user-friendly behavior and name registers appropriately.
If this ends up being needed in more places we can reconsider.